1. Field of the Invention
This invention relates to the field of spectroscopy and more specifically to a multi-pass reflective cell for spectroscopy.
2. Background of the Invention
Spectroscopy involves measuring the absorption or radiation of energy by a substance. Typically, gas cells have been used for spectroscopy measurements. The conventional gas cell includes an electromagnetic radiation source optically aligned with a detector. A sample gas is passed through the gas cell with electromagnetic radiation from the source passing through the sample gas in the gas cell to the detector. One potential drawback to this type of measurement is that some substances may exhibit a poor instrumental response either due to the concentration of the substance of interest or due to its lack of interaction with the source energy, which may lead to the inability of the detection system to be able to make an adequate measurement due to the path of the electromagnetic radiation within the gas cell not being of sufficient length. This result may be a consequence of the Beer-Lambert law of spectroscopy as represented by the following equation:−I=I0e−abc,where I is the final light intensity that reaches the detector, I0 is the incident light from the optical source, a is the molar absorption coefficient that is specific to each sample, b is the pathlength of the light that interacts with the sample, and c is the concentration of the substance in the gas cell.
To overcome such drawbacks, gas cells have been developed with an increase in the length of the electromagnetic radiation interaction path within the gas cell. The increase in length has been accomplished by lengthening the overall tube and thereby increasing the optical pathlength. A disadvantage of this development includes the requirement of a large tube, which may limit the applicability of the device in many instances. In other cases, gas cells (i.e., multi-pass cells) have been developed with mirrored surfaces within the cell body that reflect the electromagnetic radiation in either a circuitous or oscillatory manner within the cell, which may cause the incident optical energy to interact With the substance of interest one or more times. Drawbacks to such gas cells include requirements that the source and detector must be optically aligned since the measurement efficiency of the device is often directly related to the efficiency of the optical energy transfer from the source to the detector. For this same reason, in the case of multi-pass cells, the mirrors used to reflect the light back and forth must be precisely aligned depending upon the overall pathlength desired, which may lead to design difficulties and performance degradation due to any optical misalignments caused by impacts or optical slippage that may occur during normal usage of the device. Further drawbacks include the inefficiencies involved with cables that connect the source and the detector to a circuit board such as a printed circuit assembly. In many optical devices using gas cells, the position of the optical elements often preclude the utilization of multiple circuit boards in order to facilitate the design, which may lead to additional complexity for the overall instrument system.
Consequently, there is a need for an improved gas cell for use in spectroscopy measurements. Further needs include a gas cell with an increased radiation path length. Additional needs include a gas cell that does not require optical alignment of the source and detector. Moreover, needs include the ability to shorten or eliminate electrical connections such that the associated source and detector electronics may be simplified.